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Recommendation issued to: R20000045

Recommendation details
Output No: R20000045
Date issued: 22 December 2000
Safety action status:
Background:

SUBJECT - BLEED AIR WARNING SYSTEM INSTALLATION IN RAYTHEON (BEECH) TURBOPROP AIRCRAFT

SAFETY DEFICIENCY

Incorrect bleed air warning installation can lead the crew to mis-identify a bleed air duct leak and isolate the wrong system leading to pressurisation failure.


FACTUAL INFORMATION

Related Occurrences

199704165

During climb through FL180 the left bleed air warning light of the Raytheon (Beech) B200 illuminated. The left bleed air system was turned off when the aircraft was stabilised at cruise at FL220. The cabin then slowly depressurised to 14,000 ft. A descent was initiated and the aircraft returned to the departure aerodrome.

A subsequent maintenance investigation found loose ducting and a hole in the right bleed air system. In addition, it was found that the left bleed air pressure transmitter was connected to the right air bleed system and vice versa. The Australian Transport Safety Bureau advised the Civil Aviation Safety Authority of these findings when they became known.

The pilot in command subsequently stated that if the uncontained bleed air leak had led to loss of the aircraft, the investigation may have attributed this to his actions in not properly identifying and dealing with a bleed air leak. This would have been the case even though he had correctly identified and shut off the system annunciating the fault. He reasoned that the cross-connected warning system led him to shut off the serviceable bleed air system. This then left the ruptured and leaking system still in operation jeopardising the safety of the aircraft. If this had resulted in an accident, the reason the pilot selected the wrong system off may have been impossible to determine from examining the wreckage and may have been incorrectly attributed to misidentifying the failed system.

ANALYSIS

The B200 bleed air warning system is provided to give a visual warning to the crew of a leak in the airframe bleed air ducting behind the firewall of either engine. The bleed ducting is designed to contain hot bleed air, which is under pressure. To protect surrounding airframe structure from radiated heat the pipes are wrapped in heat insulation lagging, which effectively prohibits visual inspection of the piping for defects, such as pinholes, and fatigue type cracks.

To monitor these pipes for leakage, a small diameter plastic ethylene vinyl acetate (EVA) tube is routed in close proximity for the entire length of each bleed ducting. This EVA tube is connected to a normally open pressure switch located under the cabin floor. A pressure of approximately 18 psi is supplied to this EVA tube continuously from the de-ice manifold. When hot air from a ruptured bleed air pipe contacts the EVA tube, the tubing melts. Depending on which half of the system has failed, the released pressure closes the pressure switch and illuminates the appropriate red left bleed air fail (LH BL AIR FAIL) or right bleed air fail (RH BL AIR FAIL) annunciator in the cockpit. The warning informs the crew that there is a hot air leak somewhere in the left or right ducting, which can then be isolated from the aircraft system by appropriate checklist actions.

The EVA tubes, one for the left bleed air supply ducting and one for the right, are routed forward to the pressure switches under the cabin floor. The tubing from each side is identical. The pressure switches are mounted side by side and are identical in appearance and function.

It is possible that during the replacement of either the tubing or the switches, the EVA tubing could be crossed and the indication system would become cross-connected, displaying warnings for the wrong half of the bleed air supply system plumbing. The crew can then be misled into shutting off the remaining serviceable bleed air source. This would result in a loss of cabin pressurisation and the continued danger of leaking bleed air in a wing cavity. That was the situation that presented itself in this occurrence.

In this situation with the bleed air system now completely disabled, the pilot in command and passengers would be put at additional risk of loss consciousness through the loss of pressurisation after unknowingly turning off the remaining serviceable system.

The pressure source for the EVA tubes, the de-ice manifold, is pressurised by either engine operating and supplies both left and right tubes. There is no provision for the pilot to test the warning systems individually to ensure the warning generated is correct.

An examination of the maintenance manual for the B200 series aircraft revealed that the bleed air warning system does not have any functional test procedure listed to verify the correct function of the bleed air warning system after maintenance.

This type of bleed air warning system is used on, but not limited to the B200 aircraft within the Raytheon product line, therefore other models including the B300, B350 and B1900 could also be affected.

Output text

The Australian Transport Safety Bureau recommends that the Australian Civil Aviation Safety Authority liaise with the United States Federal Aviation Administration to ensure that this safety deficiency is addressed by the manufacturer.

As a result of the investigation of this safety deficiency, the Bureau simultaneously issues the following recommendations:

R20000042

The Australian Transport Safety Bureau recommends that Raytheon Aircraft review the Engine Bleed-Air Warning System post maintenance procedures to include a functional test of the system to verify that the failure warning correctly annunciates the failed system.

R20000043

The Australian Transport Safety Bureau recommends that Raytheon Aircraft ensure some positive means of preventing the possibility of system cross-connection of the bleed air failure pressure transmitters.

R20000044

The Australian Transport Safety Bureau recommends that the United States Federal Aviation Administration monitor the aircraft manufacturer to ensure that this safety deficiency is addressed.

R20000284

The Australian Transport Safety Bureau recommends that the Australian Civil Aviation Safety Authority alert all owners and operators of Raytheon (Beech) turboprop aircraft fitted with the EVA tube type bleed air warning system, to adopt post maintenance procedures which include a functional test of the system to verify that the failure warning correctly annunciates the failed system.

Initial response
Date issued: 01 March 2001
Response from: R20000045
Action status: Closed - Accepted
Response text:

Thank you for your letter dated 22 December 2000 concerning Air Safety Recommendations R20000045 and R20000284.

CASA accepts both Air Safety Recommendations and the Maintenance Standards Branch is in the process of preparing an Airworthiness Advisory Circular to raise the awareness of the operators recommended by R20000284. As part of this process, the Technical Specialist concerned will liaise with the Federal Aviation Administration to confirm their action with respect to the manufacturer.

 
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Last update 05 April 2012